As an electron source used for an electron beam device such as an electron microscope, there are a thermal electron source, a field emission electron source, a Schottky electron source, and the like. Energy diagrams demonstrating operation principles of the electron sources are respectively illustrated in FIGS. 1A to 1C. The thermal electron source illustrated in FIG. 1A heats a filament of tungsten (W) processed into a hairpin shape to about 2800 K and takes out an electron e into a vacuum by causing the electron thermally excited in W solid to cross a barrier of work function Φ (4.3 to 4.5 eV) of W. Since the electron source is always heated, it is possible to take out a stable electron beam with no contamination on the surface of the electron source and little current fluctuation. On the other hand, since the electron source is heated to a high temperature, an energy half width ΔE of emitted electrons is as wide as 3 to 4 eV, and electrons are emitted from the entire heated portion and thus, an electron emission area α is wide and brightness B (amount of emitted current per unit area or per unit solid angle) is as low as 105 A/cm2 sr. For that reason, a hexaboride electron source such as LaB6 whose work function Φ is 2.6 eV lower than that of W is also used. The LaB6 thermal electron source can reduce an operating temperature to about 1700 to 1900 K because the work function Φ of the LaB6 thermal electron source is low. Therefore, the energy half width ΔE can be suppressed to 2 to 3 eV, and brightness B can also be raised to about 106 A/cm2 sr. In PTL 1 and PTL 2, thermal electron sources which heat the hexaboride to emit thermal electrons are disclosed. These electron sources are used, for example, as electron sources for low-resolution but easy to handle and inexpensive simple electron microscopes.
The field emission electron source illustrated in FIG. 1B is used as an electron source for a high resolution electron microscope because the field emission electron source has high mono-chromaticity and can emit a high brightness electron beam. A tungsten (W) tip with a sharpened tip is widely used as the field emission electron source. A high electric field is applied by concentrating an external electric field E to a tip end of the W tip, and the electron e in the W tip is quantum mechanically transmitted through an effectively thinned vacuum barrier and released into the vacuum. Since the field emission electron source can operate at room temperature, the energy half width ΔE of the electron e is as narrow as about 0.3 eV and electrons are emitted from a narrow electron emission area α of a very sharp tip end. Therefore, the field emission electron source is characterized by high brightness of 108 A/cm2 sr. In order to further narrow the energy width ΔE and increase brightness B even in the field emission electron source, a field emission electron source using a nanowire of hexaboride such as LaB6 having a low work function Φ has also been proposed (for example, PTL 3). Since LaB6 has a lower work function barrier than W, it is possible to further reduce the half energy width ΔE at which field emission can be caused by allowing electrons to transmit in a lower electric field.
On the other hand, as illustrated in FIG. 1C, a Schottky electron source in which zirconium oxide (ZrO) is coated on a W tip is used in a length measurement scanning electron microscope for measuring dimensions of a semiconductor device. The Schottky electron source is always heated to about 1800 K, and in which ZrO thermally diffused to the tip end of the W tip lowers the work function Φ of the surface of the W tip to about 2.6 eV and thermoelectrons are emitted beyond the barrier (Schottky barrier) of the work function Φ reduced by the external electric field E applied to the tip end of the W tip and mirror image potential. The Schottky electron source can stably take out a larger current than that of the field emission electron source, but since the operating temperature is high, the half energy width ΔE becomes as large as about 0.5 to 1 eV.